1. Field of the Invention
The present invention relates to a metal film-forming method which is useful for forming a metal film, such as a copper interconnect film for circuit interconnects, on a surface of a substrate such as an electronic circuit substrate.
2. Description of the Related Art
Because of advantageous such as low interconnection resistance, copper is frequently used these days as an interconnect material for an electronic circuit substrate. A copper interconnect film is generally formed by plating. In the formation of a copper interconnect film on a surface of a substrate by plating, it is common practice to form a base metal film, such as tungsten, titanium, tantalum or ruthenium, on the surface of the substrate prior to plating in order to feed electricity to the entire substrate and prevent a reaction of copper with a base material upon plating.
When a base metal film, after its formation, is allowed to stand in the air, a natural oxide film will be formed on a surface of the base metal film. When copper plating is carried out on a surface of such a base metal film with a natural oxide film formed thereon, a copper plated film may not be formed or, if formed, the adhesion strength of the copper plated film to the base metal film will be low. In particular, nowadays when the width of interconnects is becoming smaller, and thus the contact area between interconnects and a base metal film is becoming increasingly smaller, a low adhesion strength between a copper plated film, constituting interconnects, and a base metal film may lead to high resistance of the interconnects and even to no passage of electric current. To secure a sufficient adhesion strength between a copper plated film and a base metal film is therefore becoming an increasingly important problem.
Studies are therefore being conducted to remove a natural oxide film, which has been formed on a surface of a base metal film such as tungsten, titanium, tantalum or ruthenium, e.g., by a method which involves electrolytic treatment of the base metal film in an electrolytic solution or a method which involves reduction treatment of the base metal film with hydrogen gas. However, there is time restriction from the removal of a natural oxide film by such a method until the start of copper plating; and a complicated process or apparatus is necessary to carry out the removal of a natural oxide film and copper plating successively. It is desirable in terms of process control and apparatus construction if the formation of a metal film having strong adhesion to the surface of the base metal film can be carried out with ease.
A method which involves heat decomposition of copper formate has been proposed as a method to deposit copper on a surface of a resin substrate, e.g., made of an epoxy resin, thereby forming a copper film (see Japanese Patent Laid-Open Publication No. 2008-111093). In this method, the formation of the copper film on the surface of the substrate is carried out in an inert gas atmosphere into which ammonia gas is mixed. Because of the reducing power of ammonia gas, this method is considered to be capable of depositing copper through heat decomposition of copper formate while removing a natural oxide from a surface of a base metal film. Ammonia gas, however, is a deleterious substance, and therefore measures need to be taken for supply and disposal of ammonia gas, which necessitates a complicated treatment facility.